Solvent refining is a well established processing tool for refiners and includes liquid-liquid extraction, solvent dewaxing, propane deasphalting, and modifications of these processes. Solvent refining is a petroleum fractionation procedure that deals with liquid phases of complex compositions and solubility equilibria under various conditions of temperature, mixing, concentration, and other factors.
Liquid-liquid solvent extraction has been practiced for almost 75 years. Liquid sulfur dioxide solvent has long been used to treat kerosene to recover a paraffin enriched raffinate and an aromatic enriched extract. Other well known solvents used in liquid-liquid solvent extraction include furfural, phenol and diethylene glycol. Some solvents can be modified with water to change their selectivities for certain components in the hydrocarbon feedstocks charged to the extraction process.
Catalytic cracking fractionation products, including such liquids as light cycle oil, heavy cycle oil and decant oil, are commonly subjected to solvent extraction processes. By contacting the catalytic cracking fractionation products with liquid sulfur dioxide solvent in a solvent extraction column, both a paraffin enriched raffinate and an aromatic enriched extract may be obtained. The aromatic extract has a high Bureau of Mines Correlation Index (BMCI) and is a desirable feedstock for oil furnace-type carbon black manufacture. As is known in the art, the more aromatic feedstocks produce more carbon black per gallon of feedstock and the carbon blacks produced have higher structure values than corresponding nodule-size carbon blacks produced from the less aromatic or lower BMCI oils. BMCI is defined as follows: ##EQU1## where K=50% ASTM Boiling Point in .degree.K. (.degree.K.=.degree.C.+273.1.degree.)
G=Specific Gravity, 60.degree. F./60.degree. F.
In order to produce an aromatic extract oil product of a preselected BMCI value as feed for carbon black manufacture, it is necessary to control the solvent extraction process. Prior operations have used the level of extract solution in the solvent extraction tower to manipulate the rate of withdrawal of the aromatic extract phase. This phase is then charged to the solvent stripper or solvent recovery tower from which the extract oil product is yielded. This method does not give direct control over the BMCI of the aromatic extract oil product. It has been found that an aromatic extract oil product of a preselected BMCI can be produced by controlling the rate of removal of the aromatic extract phase from a solvent extraction zone in response to the specific gravity (or API gravity) of the aromatic extract oil product. If the BMCI of the aromatic extract oil product is too low, this will be reflected in terms of a low specific gravity product. By decreasing the aromatic extract oil removal rate the BMCI will increase. Similarly if the specific gravity is too high, indicating a higher than desired BMCI, the aromatic extract oil removal rate should be increased.
Accordingly, it is an object of this invention to produce an aromatic extract oil of a predetermined BMCI.
A further object of this invention is to provide a method for controlling the rate of removal of aromatic extract oil from a solvent extraction zone and in response to the specific gravity of the aromatic extract oil.
These and other objects will be made apparent from this disclosure of the invention and the appended claims.